Freezing Fridge
Research to the influence of phase change materials in a fridge.
Thijs Jansen
Commissioned by Flexines
Course: International Power Generation and Distribution
Institute of Engineering
Hanze Hogeschool Groningen
Groningen, June ’10
Foreword
For my graduation, I had a choice of assignments at different companies.
When I heard about the project Flexines and the assignment with the fridge, I was immediately enthusiastic.I find it very interesting and fun to be working on new innovations in the field of energy.
I have enjoyed working on this project. Because it was not only electrical but also physics.
It was also nice to work with the colleagues within the project Flexines who occasionally gave good advice.For that I thank them.
Groningen, June2010
Thijs Jansen
Summary
Electrical energy is increasingly produced in a sustainable way.
The disadvantage is that this kind of generation is highly depending on the weather conditions and therefore not always present at times when we need it and the other way around.
Purpose is to identify ways to save energy at times when it is there but we do not need it.
This research examined the extent to which energy can be stored using a thermal buffer in a refrigerator and thus the time of switching on and off. With this study, two kinds of Phase change materials (PCM) are used as a thermal buffer. These materials are water with a phase change temperature of 0°C and an organic solution with a phase change temperature of 4°C.
With an ambient temperature of 20°C, these two materials showed that the temperature difference between the PCM and the compartment of the fridge is 10°C when the temperature in the fridge reaches a constant level. The problem with these two materials is that their phase change temperature is to high. With water the temperature in the fridge stays constant at 10°C and for the organic solution this is at 14°C to 15°C. Because the temperature in the fridge has to be 5°C to 7°C a PCM with a phase change temperature of -4°C is needed.
The range of extending the time between turning on and off the refrigerator is depending on the amount of material that is used. The time can be extended from 1 hour without PCM till up to eight hours with 1,5 liters of PCM.
The use of PCM’s will not only help to extend the time of on and off but it also makes the fridge more efficient. In this study the efficiency of the fridge became 3.3% better.
To prevent the temperature in the fridge getting too cold, the PCM needs to cover the whole surface of the cooling element. Otherwise, the cooling element has more influence on the temperature in the refrigerator than the PCM. Therefore the temperature will drop beneath 0°C before the whole phase change is completed.
Due to time constraints in this study practical research on the results of a PCM with a phase change temperature of -4°C has not been performed.
However, a simulation model of the fridge is made which gives a very good presentation of reality. This model also shows that a material of -4°C is best to use.
1Table of content
2Introduction
2.1Background
2.2Phase Change Materials (PCM’s)
2.2.1Organic solutions
2.2.2Eutectics
2.2.3Salt solutions
2.3Research assignment
2.4Plan of approach
3The fridge
3.1What is a fridge
3.2Values
3.2.1Average heat flow
3.2.2Resistance fridge
3.2.3Capacity fridge
3.2.4Full fridge
4Measurements
4.1Temperature sensors
4.1.1Application
4.1.2Type
4.1.3Calibration
4.2Phidgetboard
4.3Amplifier
4.4Energy consumption
4.5Measurements fridge
4.5.1Average temperature
4.5.2Influence products
4.5.3Power
4.5.4PCM
4.5.5Energy consumption of the fridge
5Vissim model
5.1Turning the fridge on and off
5.2Heat flow of the cooling element
5.3Heat flow PCM
5.4Heat flow compartment fridge and products.
5.5Evaluation
5.6PCM of -4ºC
6Conclusion
References
Attachment 1. Temperature range organic solutions
Attachment 2. Temperature range eutectics
Attachment 3. Measurement protocol.
Attachment 4. Datasheet LM35.
Attachment 5. Calibration Amplifier.
Attachment 6. Vissim model
2Introduction
2.1Background
This project “Freezing Fridge” is part of a bigger project called Flexines. Flexines is acollaborative project of ECN, RuG, Kema, TNO and HG. The goal of this project is to balance the supply and demand of energy. The World is running out of fossil fuels so we will make use of sustainable energy more and more.The amount of sustainable energy is highly depending on the weather conditions which is very unpredictable.In the futurethe energy supply will be irregular while the demand of energy will not automatically change its behavior to match the supply.
The demand of energy is controllable between certain limits. Under special circumstances some devices could havethe ability to regulate their energy consumption by using a thermal buffer system to store energy.Thereby their energy usage could be shifted in time.In this case we speak of timeshiftingof energy demand. The project “Freezing Fridge” is a research project to see what is possible with a fridge regarding this time shifting by making use of phase change materials.
2.2Phase Change Materials (PCM’s)
One of the biggest energy users in our houses is the fridge. The fridge uses the energy in a short period of time. Thereby it is a device which is not easy to shift in time. PCM’s are a valuable tool for thermal buffers with an extremely low increase of volume. This means that the time shift can be increased. PCM is a discipline which is quite new. It appears that application of these materials for smart grids (i.e. matching supply and demand of energy) is an extremely important value.
We all know the phenomenon of water. We find it very natural that water turns into ice at a temperature of 0°C. Phase change materials are materials that in many respects are similar to water. Phase change materials have the same property as water, they change phase at a certain temperature. The only difference between water and phase change materials is that these materials are available in different configurations. This means the temperature at which the phase change takes place can be selected.
For the phase change from liquid to solid a lot of energy is needed in the form of cold while the temperature of the material remains the same until the phase transition from liquid to solid is fully completed. After that the temperature of the material will go further down.
The other way around it takes the same amount of energy to go from solid to liquid again. And again the temperature will be the same during the phase change. The only difference is that the “cold” energy is now not entering the material but it is extracted from it by the environment. The amount of energy to be added or removed from the material for a complete phase change is called latent heat and is given in kJ/kg or MJ/m3.
The different types of compounds that are currently used to make phase change materials can be divided into three groups, namely:
- Organic solutions
- Salt Solutions
- Eutectic
These three groups are all made of different materials and therefore each has its own specific properties. These different properties each have their advantages and disadvantages.
One of the different properties between the materials is that they each have a different range of temperatures for which they are available as phase change material.
Table 2.4.1 shows the different ranges at which the materials are available.
Minimum temperature / Maximum temperatureOrganic solutions / 2°C / 164°C
Salt solutions / 7°C / 117°C
Eutectic / -173°C / -2°C
Table2.4.1. Temperaturerange different phase change materials.
2.2.1Organic solutions
Organic solutions are mostly made of paraffin. Paraffin has some advantages and disadvantages.
For instance it freezes with almost no change of super cooling. Super cooling means that a liquid can be cooled down below its freezing point without changing in to its solid state.
It has a very sharp phase change temperature so there is no big temperature change during the phase change.
Organics solutions have no segregation, even after a long period of time. So the material will be able to freeze and melt for many times without losing its capacity.
One of the disadvantages of the material is that in the solid state the material has a low thermal conductivity. Also the latent heat capacity per volume is not very high.
Attachment 1 shows the complete range of available temperatures for organics with corresponding heat properties.
2.2.2Eutectics
Eutectics consist of a mixture of two or more substances. These substances are mixed in such a way to provide the desired melting/freezing point. Each substance has its own phase change temperature.Combining these materials gives a lower phase change temperature than the substances had on their own.
Eutectics have an even sharper phase change temperature point then organics. Eutectics melt and freeze completely at the designed temperature.The latent heat capacity of eutectics is a little higher than with organics.Attachment 2 shows the complete range of available temperatures for eutectics with corresponding heat properties.
2.2.3Salt solutions
Salt solutions are mostly bases on Glauber’s salts or sodium sulphate decahydrates.
These salts are available on a very large scale which makes them very cheap compared with the other two. The major problem with salt solutions is that every cycle of freezing and melting, the salts have the tends to crystallize in such a way that they get separated from the saturated solution they. After melting these crystals sink to the bottom of the substance and with the next frozen they will not be able to recombine with the rest of the substance. This will result in a continual loss of capacity performance.
Salt solutions also have tendency to super cooling. And there change of volume between liquid en solid is big compared withorganics and eutectics.
2.3Research assignment
The goal of this project is to do research to the influence of PCM’s when they are installed in a fridge.
Important research questions are:
• How do PCM's behave in this application.
• What is the extra room for negotiation, i.e. how we can improve supply and demand matching with these materials?
• What effects do the PCM’s have on the energy consumption of the fridge?
2.4Plan of approach
First the behavior of the fridge without PCM’s is determined.This isdone to see what the differences are between the fridge with and without PCM’s.To be able do to this, temperature sensorsare build. Together with a program to read and store the measurements from the sensors.
During these measurements, information about the PCM’s is collected.What kind of materials are there and what properties does each one have? With these information and the measurements a good decision can be made for which one is best to use.
As soon as the PCM’s are ordered, time is used to research what would be the best place in the fridge to attach the PCM’s. In this phase of the project a simulation program of the fridge is made to simulate the fridge.This simulation willhelp to make a prediction of the effects of the PCM’s.
After the PCM’s are delivered, they will be placed in the fridge and new measurements can start.
After these measurements the results have to be evaluated to see what the effects are and if this is in line with the simulation program.
In this phase the advantages and disadvantages of this system will be evaluated. The energy consumption of the fridge is part of this.
3The fridge
3.1What is a fridge
A fridge can be seen as a device that transports heat.
The compressor of the fridge takes out the heat inside the fridge by a cooling element.
When the fridge is off, the environment heats up the fridge again. The speed of cooling down and heating up is depending on the heat capacity of the different elements in the fridge and the resistance that the heat flow undertakes. This process can be compared with an electrical circuit.
In an empty fridge, the heat undertakes three resistances. These are:
R1The resistance from the environment to the compartment of the fridge.
R2The resistance from the compartment of the fridge to the cooling element of the fridge.
R3The resistance from the environment to the cooling element of the fridge.
An empty fridge has two capacities:
C1The compartment of the fridge.
C2The cooling element of the fridge.
For the electrical circuit there is a voltage over the resistors and a current goes through.
In this case, the voltage is the temperature difference and the current is the heat flow.
Underneath is a diagram of an electrical circuit which represents an empty fridge.
Figure 3.1.1. Electrical diagram of an empty fridge.
3.2Values
To make a good model of the fridge we have to determine the values of each component.
Once the fridge is running, it turns on and off at the same temperature every time.
If we measure the energy that is used to cool the fridge, we know that at the time the fridge will turn on again, the energy (heat) that was extracted from the fridge has now returned back into the fridge from the environment. So the capacities are unloaded and loaded again. These capacities won’t consume energy, they only store it for a while. So for a hole period of cooling down and heating up again we can say all the energy that went into the fridge also came out. All this energy went through the resistors during the whole period.
3.2.1Average heat flow
With the first measurement, the temperature is measured on three places.
-The temperature of the environment (point 1 in the diagram)
-The temperature of the compartment on several places (point 2 in the diagram)
-The temperature of the element (point 3 in the diagram)
With the total energy that entered the fridge and the time that it took, the average heat flow can be calculated. A measurement of ten cool down and heat up cycles is used. At first the electrical consumption of the fridge is determined. With this energy consumption, the average power during cool down is calculated.
Electrical consumption of the fridge:
Amount of energy in heat to fridge:
This is the electrical power of the fridge. To get the power in terms of heat this has to be multiplied with the COP of the fridge. With the amount of energy which is subtracted from the fridge during cool down, the average heat flow of the fridge warming up again can be calculated by dividing this over the total time period.
3.2.2Resistance fridge
With the average heat flow and the average ΔTemperature between the fridge and the ambient the total the total resistance of the fridge can be calculated.
This is the total resistance of the fridge.
To divide this resistance in to three separated resistances, the difference in temperature between the ambient, the compartment of the fridge and the element of the fridge are used. To be able to solve this question I assumed the material at the back of the fridge is the same as the material used at the other surface of the fridge. Otherwise this problem can’t be solved.
These resistances are the total resistances of the different surfaces.
To calculate the specific thermal resistance of the materials, these numbers have to be multiplied by the surface.
Specific thermal resistance:
3.2.3Capacity fridge
Now that the resistances are known, the capacities of the fridge can be calculated.
To do this one period of the measurement which was used to calculate the average heat flow is used.
For this period the heat flow between the different parts of the fridge is calculated for every moment. These heat flows be expressed in a formula.
The graphic below shows the heat flow for the element with the formula. On the next page you will find the heat flow of the compartment of the fridge.
Figure 3.2.3.1. Total heat flow of the element of the fridge
From the thermodynamics we know that:
Rewriting these formulas we are able to calculate the capacity of the fridge:
Figure 3.2.3.2. Total heat flow of the compartment of the fridge.
For the compartment of the fridge:
Now all the components of the fridge are determined. These components are used to make the simulation model in Vissim.
3.2.4Full fridge
When you fill the fridge with products or PCM’s, figure 3.1.1. will change and look like figure 3.2.4.1.
For each product its own capacity and resistance has to be determined. This also count’s for the PCM.
Figure 3.4.2.1. electrical diagram of a loaded fridge.
As you can see R2 is still in the same position but due to the fact that the PCM is attached to the back of the fridge, the resistance from the element directly to the compartment of the fridge will be bigger because the surface of the element directly to the compartment will decrease with the surface of the PCM covering the element.
4Measurements
For this project I made use of a fridge of ZANUSSI,type ZRG 616 CW, model TT 160 C.
To determine the behavior of the fridge I had decided to collect seven measurements, which are:
-Temperature in the fridge on four different places.
-Temperature of the PCM.
-Ambient temperature of the fridge.
-The energy consumption of the fridge.
All the data is collected per time unit.
To determine the amount of power the fridge uses, these measurements are done in seconds.
Once this is done the measurements will take place in minutes.
Because a fridge responds different when it’s empty than when it is full, different scenariosare tested. This is done by the measurement protocol (attachment 3)
4.1Temperature sensors
4.1.1Application
The fridge has three sections separated by glass, therefore each Floor gets its own sensor. Also the temperature of the cooling element is measured.This way there is a good representation of the temperatures in the fridge. Because the temperature change in the fridge is strongly depending on the ambient temperature, this is also measured.